Metal caps with plastisol liners popularly known as press-on twist-off or P-T caps are widely used by vacuum packagers largely because of the capping speed and economy possible and the seal integrity of such closures. Such closures depend on the thread formation in the cap by the compression of the liner by the container neck thread at the elevated temperatures used for such a closure system. However, such closures present problems in removal because of the intimacy of the thread engagement which is formed in situ. The fact that such caps are ordinarily used with vacuum and that metal cap shells offer a poor gripping surface adds to the difficulty of their removal. An additional problem associated with such closure is the difficulty of rethreading them on reclosure because the in situ method of their formation creates too intimate a thread engagement.
Attempts to duplicate or improve on the advantages of low cost, high speed capping with a one piece press-on twist-off plastic cap have been unsuccessful largely due to problems associated with the inability to achieve a cap skirt having the formability characteristics during the capping process and the mechanical characteristics to provide a positive thread engagement suitable to maintain the closure and sealing integrity while capped and to provide thread definitions with sufficient strength for removal by twisting.